Motion detection using capacitor having different work function materials

ABSTRACT

An apparatus for detecting mechanical displacement in a micro-electromechanical system includes a capacitor having first and second plates spaced from one another, the first and second plates having different work functions and being electrically connected with each other. The capacitor plates are movable with respect to one another such that a spacing between the plates changes in response to a force. A current through the capacitor represents a rate of change in the spacing between the plates at a given time.

FIELD OF THE INVENTION

The present invention relates to methods and apparatuses for detectingmotion.

BACKGROUND OF THE INVENTION

In modern video game controllers, for example, the conventional way todetect motion is by measuring capacitance. One such motion detector 10based on capacitance measurements is shown in FIG. 1. The motiondetector 10 includes two metal plates 12, 14 separated by a smalldistance D. One of the plates is fixed and the other is free to moverelative to the other in the direction illustrated by the arrow inresponse to some force. Some form of spring, such as bellow or diaphragm16, is disposed between the two metal plates 12, 14 to restore theplates to their static relationship after a force induced motion of thetwo plates relative to one another. If there is a motion, the distancebetween the two metal plates 12, 14 will change from D to D′. The slightdifference between D′ and D will result in a change in the capacitancebetween the two metal plates. By measuring the capacitance during thecourse of motion, the acceleration, the velocity and the distance ofmotion can be calculated. The motion sensors in Nintendo's Wii® gamecontroller are based on this principle.

The capacitance of the capacitor 10 is C=x*y*∈/D, where x and y are thetwo dimensions of the metal plates 12, 14, ∈ is the permittivity of thedielectric between the plates 12. 14, and D is the distance betweenplates. With these types of prior art motion detectors, the change incapacitance is very small and hard to measure in real time. Analternative motion sensor that is faster, cheaper, and/or more accurateis desired.

SUMMARY OF THE INVENTION

An apparatus for detecting mechanical displacement in amicro-electromechanical system includes a capacitor having first andsecond plates spaced from one another, the first and second plateshaving different work functions and being electrically connected witheach other. The capacitor plates are movable with respect to one anothersuch that a spacing between the plates changes in response to a force. Acurrent through the capacitor represents a rate of change in the spacingbetween the plates at a given time.

The above and other features of the present invention will be betterunderstood from the following detailed description of the preferredembodiments of the invention that is provided in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of theinvention, as well as other information pertinent to the disclosure, inwhich:

FIG. 1 illustrates a capacitor for use in a prior art motion sensor;

FIGS. 2( a) to 2(d) illustrate capacitors for use in a motion sensoraccording to embodiments of the present invention;

FIG. 3 illustrates capacitors oriented along three axes for motiondetection in three dimensions;

FIG. 4 illustrates an embodiment of a current sensor for capacitor;

FIG. 5 illustrates an embodiment of a differential sensing circuit; and

FIG. 6 illustrates a structure having interdigitated capacitors forsensing movement.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. In the description, relativeterms such as “lower,” “upper,” “horizontal,” “vertical,” “above,”“below,” “up,” “down,” “top” and “bottom” as well as derivative thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should beconstrued to refer to the orientation as then described or as shown inthe drawing under discussion. These relative terms are for convenienceof description and do not require that the apparatus be constructed oroperated in a particular orientation. Terms concerning attachments,coupling and the like, such as “connected” and “interconnected,” referto a relationship wherein structures are secured or attached to oneanother either directly or indirectly through intervening structures, aswell as both movable or rigid attachments or relationships, unlessexpressly described otherwise.

A method and apparatus for detecting mechanical displacement inmicro-electromechanical (MEMS) devices is provided. The displacementdetermination is based on the detection of current generated bymodulating the distance between two plates of a capacitor where thecapacitor plates are made of materials with dissimilar work functions.By “work function” it is meant the minimum amount of energy required toremove an electron from the surface of a conducting or semi-conductingmaterial. If the plates are electrically connected to one another, thework function difference between the materials forming the two capacitorplates generates a built-in electrical field. The electric field acrossthe capacitor is modulated with the distance between the plates. Thatis, at equilibrium there will be an electric field built-in between theplates and no current flowing in the system. If the distance betweenthese two plates changes, the field will change and the current willflow. By measuring the magnitude of the current, the change in distancebetween the metal plates can be calculated. The current through thecapacitor represents the rate of change in spacing between the capacitorplates, i.e., plate velocity. The acceleration can also be calculatedfor a MEMS motion sensor. The sensor disclosed herein can have farranging applications, such as in video game controllers and joy sticks,accelerometers, gyroscopes, safety devices (such as air bag deployment)and others.

The capacitor plates can be formed between metals with dissimilar workfunctions, metal/semiconductor structures with dissimilar workfunctions, semiconductor materials with dissimilar work functions,between semiconductor materials with different dopants, or combinationsof these materials or structures. FIGS. 2( a) to 2(d) illustrate variousembodiments of capacitors having capacitor plates having different workfunctions. FIG. 2( a) illustrates a capacitor 100A including a firstcapacitor plate 110A and second capacitor plate 120A. The plates arespaced a distance D from one another. The dielectric between thecapacitor plates 110A, 120A is assumed to be air or vacuum. Eachcapacitor plate 110A, 120A is formed from a metal layer 112, such as Al,W, Ti, Au or other metals suitable for integration in a specific MEMSprocess flow and a semiconductor layer formed thereon. The capacitorplate 110A includes layer 114 of an N+ doped semiconductor material(e.g., silicon) and the capacitor plate 120A includes layer 116 of a P+doped semiconductor material (e.g., silicon). The capacitor plate's workfunction is determined by the work function of the material facing intothe “gap” between the plates 110A, 120A (i.e., by the N+/P+ layers 114,116). The metal layers 112 can be provided to provide a specific workfunction (if facing the gap between the plates) or to provide a lowresistance connection as shown in FIG. 2( a).

The capacitor plates 110A, 120B are physically connected to one anothervia an elastic element shown as elastic element 140. If there is noexternal force, the capacitor plates stay at a static position. If thereis relative movement between the plates, the elastic element 140 helpsrestore the plates to their static position. The elastic element 140 maybe a spring, an elastic sheet, elastic diaphragm or the like. One of theelectrode plates may be a stationary electrode plate (i.e., fixed) andthe other electrode plate may be considered a movable electrode platepositioned parallel to the stationary plate and free to move in adirection orthogonal to the major surface of the plates.

The capacitor plates 110A, 120B are electrically connected to oneanother. This electrical connection is shown as connection 130 in thedrawings of FIGS. 2( a) to 2(d). When two electrically connectedmaterials with dissimilar work functions are brought in close proximityto each other, the Fermi levels of these materials line up due to theabsence of electric current between the two materials. The work functiondifference between them creates a charged capacitor and the electricfield between these features. The charge on the capacitor is equal toQ=−ΔΦ*C/e, where e is the electron charge sign, ΔΦ is the work functiondifference (in eV) between the materials and C is the capacitance of thestructure. The electric field between the features (e.g. uniform fieldin a parallel plate capacitor) is E=−ΔΦ*/De, where D is the distancebetween the plates at a given time.

If there is movement of the plates relative to one another, such thatthe spacing between the plates changes, an electrical current isgenerated. The magnitude of the current can be determined (for aparallel plate capacitor) as follows:

I=−ΔΦ*x*y*∈/e[1/D−1/D′]/Δt=A*ΔD/[Δt(D*D′)], where A is the “structure”constant and is equal to the area of the plates (x*y) times ∈/e; ∈ isthe dielectric constant (e.g., 1 for a vacuum); D-D′ is ΔD; and Δt isthe change in time. This expression shows that the detection current isproportional to the relative velocity of the two electrodes.

The relative movement in the capacitor plates will cause an AC/transientcurrent. Assuming the distance between the two plates is not very large,the current will be proportional to the velocity of the plate (ΔD/Δt).Continuously detecting velocity provides a change in velocity (Δv) overa given time period (Δt). The first derivative of velocity/time (Δv/Δt)is acceleration. As such, the capacitor can be used as a motion detectorfor an accelerometer.

It should be noted that it is not necessary to know the physicalproperties of the elastic element that attaches the first and secondcapacitor plates in order to determine velocity and acceleration, butknowing the physical properties of the elastic element can help provideinformation on the sensitivity of the motion detector since ΔD isdependent on the properties of the elastic element. In embodiments, themaximum value of ΔD may be between around 1 μm to a few microns.

FIG. 2( b) illustrates an embodiment of a capacitor 100B where the twocapacitor plates are semiconductor materials having different dopantkinds, e.g., N+ and P+. In the illustrated embodiment, the capacitorplate 110B is a semiconductor material doped with n-type dopants and thecapacitor plate 120B is a semiconductor material doped with p-typedopants.

FIG. 2( c) illustrates an embodiment of a capacitor 100C where the twocapacitor plates are formed from different metal materials havingdifferent work functions. Capacitor plate 110C is formed from a firstmetal and capacitor plate 120C is formed from a second metal.

FIG. 2( d) illustrates an embodiment of a capacitor 100D wherein the twocapacitor plates are formed from different semiconductor materialshaving different work functions. Capacitor plate 110C is formed from afirst semiconductor material (e.g., n- or p-doped silicon) and capacitorplate 120C is formed from a second semiconductor material (e.g., n- orp-doped germanium). Different dopants, different doping concentrations,different semiconductor base materials or combinations thereof can beused to provide materials having different work functions. By way ofexample, the expected work function difference between the N+ and P+ Siis approximately 1.1 eV. Certain metals can produce work functiondifferences greater than about 2 eV.

FIG. 3 illustrates an embodiment of a motion detector having capacitorsensors aligned along three different axes, including a vertical axis, alateral axis and a longitudinal axis. This orientation can be used toform a 3-dimensional accelerometer sensing acceleration in the directionof each of the three axes.

FIG. 4 illustrates a capacitor 100 formed as discussed above, i.e., acapacitor with two capacitor plates of different work functions andmovable with respect to one another in response to a force, connected toa current sensing circuit. More specifically, the capacitor 100 isconnected in parallel with resistor R1 for generating a first currentI1. This current I1 is then mirrored and amplified via a JFETamplification stage comprising a JFET M1, resistors R2, R3 and R4 andcapacitor C1. Resistor R4 and the JFET M1 form an amplifier. ResistorsR2, R4 bias the JFET into a desired state. Capacitor C decouples the DCbias on the gate of the JFET from the MEMS element. Resistor R1 providesa voltage drop due to the MEMS-generated current. This voltage drop issensed by the amplifier.

FIG. 5 illustrates an embodiment of a sensor with differential sensing.This embodiment employs two capacitors 100, 200. The first capacitor 100is formed in the manner described above. That is, capacitor 100 has twocapacitor plates of different work functions spaced apart from oneanother and movable relative to each other in response to a force. Thesecond capacitor 200 is identical to capacitor 100 only the twocapacitor plates are fixed relative to one another so as to provide areference current. Each capacitor is connected to the source of an NMOStransistor M2 or M3, which are biased in the on state by voltage VBIAS.Resistors R6 and R5 are connected to the drains of the transistors M2and M3, respectively. Reference current I3 is generated by the referencestage (i.e., R5, M3, capacitor 200) and transient current I4 isgenerated in the detection stage (i.e., R6, M2, capacitor 100). Voltagedrops occur across resistors R5 and R6 representative of the currents I3and I4, respectively. The resulting voltages at nodes A and B arecompared and amplified by the differential amplifier

FIG. 6 illustrates an embodiment of a motion sensor including aplurality of capacitors arranged in an interdigitated assembly 300. Theinterdigitated assembly includes first and second comb electrodesections 310, 320 that are movable relative to one another in thedirection of the arrow illustrated next to the assembly 300. Forexample, the first section 310 can be fixed whereas the second section320 is free to move relative to the first section 310, or vice versa.The first section 310 includes a plurality of capacitor plate arms 312extending from a stem or trunk section 311. These capacitor plate arms312 are formed from a material having a first work function. The secondsection 320 includes a plurality of capacitor plate arms 322 extendingfrom a step or trunk section 321. Each capacitor plate arm 322 includesa first side 323 formed from a material having the same work function asthe first work function of the capacitor plate arms 312 and a secondside 324 formed from a material having a different (second) workfunction than the first work function. Each section 324 and thecapacitor plate arm 312 that it faces forms a respective capacitor. So,the interdigitated structure includes a plurality of capacitors witheach capacitor having a first plate with a first work function and asecond plate with a second work function. As the two sections 310, 320move relative to one another, the spacing (D) between the capacitorplate sections 324 and the capacitor plate arm 312 of each capacitorchanges equally. A current is generated through each branch of thecapacitor. Electrical contacts can be made to trunk section 311 and totrunk section 312 to sense the generated current. The sum of thesecurrents can be detected and averaged across the total number ofcapacitors contributing to the detected current to provide a betterindication of the actual displacement. This larger current and changestherein are easier to detect than the smaller current through a singlecapacitor, making for a more accurate detector. It should be noted thatcapacitors are also formed between arms 312 and sections 323 of arms322. However, these capacitor plates have the same work function andthus produce zero current.

In FIG. 6, it should be understood that the one of the sections 310, 320can be part of or connected to a fixed support member attached to asubstrate. The other of the sections 310, 320 is part of or connected toa deflection member attached to the support member, such as by anelastic element as discussed above, so as to deflect relative to thesupport member. Any number of kinds of support members and deflectionmembers can be provided for positioning the capacitor described hereinrelative to one another. These support members are known in the art ofcapacitive motion detection and are used to support and allow deflectionrelative to one another of the plates of capacitors 10 described abovein the Background section.

As is conventional, the capacitor structures disclosed herein can beincorporated into a sense element chip. While not shown, a deviceincorporating the sense element chip may include an interfaceelectronics chip, a substrate, a ceramic chip carrier, a cover enclosingthese structures or other structures as will be familiar to those in theart of capacitive motion detection.

Although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly to include other variants and embodiments ofthe invention that may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

What is claimed is:
 1. A micro-electromechanical motion sensor forsensing motion of a device to which the sensor is attached, the motionsensor comprising a capacitor having first and second capacitor platesspaced from one another, the first and second plates having differentwork functions and being electrically connected with each other, whereinthe plates are movable with respect to one another such that a spacingbetween the plates changes in response to a force from motion of thedevice to which the sensor is attached, a current through the capacitorbeing representative of a rate of change in the spacing between theplates at a given time, the motion sensor further comprising a referencecapacitor having third and fourth capacitor plates spaced from oneanother, the third and fourth capacitor plates having different workfunctions, wherein the third and fourth capacitor plates are fixed withrespect to one another such that a spacing between the plates ismaintained in response to the force to provide a reference currentthrough the reference capacitor.
 2. The micro-electromechanical motionsensor of claim 1, wherein the first and second capacitor platescomprise first and second metal materials having different workfunctions.
 3. The micro-electromechanical motion sensor of claim 1,wherein the first and second capacitor plates comprise first and secondsemiconductor materials having different work functions.
 4. Themicro-electromechanical motion sensor of claim 3, wherein the first andsecond capacitor plates further comprise a conductive contact layer towhich the first and second semiconductor materials are coupled.
 5. Themicro-electromechanical motion sensor of claim 1, wherein the apparatusincludes a plurality of the capacitors oriented to detect motion along aplurality of different axes.
 6. The micro-electromechanical motionsensor of claim 5, wherein the plurality of capacitors comprises threecapacitors oriented to detect motion along a vertical axis, a lateralaxis and a longitudinal axis.
 7. The micro-electromechanical motionsensor of claim 1, wherein the apparatus includes a plurality of thecapacitors arranged in an interdigitated assembly.
 8. Themicro-electromechanical motion sensor of claim 7, wherein the apparatusincludes: a first comb electrode having a plurality of first capacitorplate arms having a first work function, and a second comb electrodehaving a plurality of second capacitor plate arms interdigitated withthe plurality of first capacitor plate arms, each second capacitor platearm having a material having the first work function on a first sidethereof facing a first capacitor plate arm and a material having asecond work function different from the first work function on a secondside thereof facing a different first capacitor plate arm.
 9. Themicro-electromechanical motion sensor of claim 1, wherein the capacitorsare connected in parallel with one another between two power supplynodes, each capacitor being coupled in series with a resistive element,the apparatus further comprising a comparator having a first inputcoupled to a first node intermediate the capacitor and the respectiveresistive element in series therewith and having a second input coupledto a first node intermediate the reference capacitor and the respectiveresistive element in series therewith, the comparator having an outputfor providing a differential output signal.
 10. Amicro-electromechanical system (MEMS) motion sensor for sensing motionof a device to which the sensor is attached, the motion sensorcomprising: a fixed support member; a deflection member attached to thesupport member so as to deflect relative to the support member frommotion of the device to which the sensor is attached; a first capacitorplate connected to the fixed support member; and a second capacitorplate connected to the deflection member, the second capacitor platebeing parallel to and spaced from the first capacitor plate, the firstcapacitor plate and the second capacitor plate forming a capacitor, thefirst and second capacitor plates having different work functions andbeing electrically connected with each other to provide a currentthrough the capacitor, the current through the capacitor being based onthe deflection of the deflection member, the motion sensor furthercomprising a reference capacitor having third and fourth capacitorplates spaced from one another, the third and fourth capacitor plateshaving different work functions, wherein the third and fourth capacitorplates are fixed with respect to one another such that a spacing betweenthe plates is maintained for providing a reference current through thereference capacitor.
 11. The MEMS motion sensor of claim 10, wherein thefixed support member and the deflection member are attached through anelastic member.
 12. The MEMS motion sensor of claim 10, wherein the MEMScomprises three capacitors oriented to detect motion along a verticalaxis, a lateral axis and a longitudinal axis, each capacitor having twospaced capacitor plates of different work functions.
 13. The MEMS motionsensor of claim 10, wherein the fixed support member comprises aplurality of the first capacitor plates and the deflection member has aplurality of the second capacitor plates, wherein the plurality of firstand second capacitor plates are arranged in an interdigitatedrelationship to form a plurality of capacitors.
 14. The MEMS motionsensor of claim 13, wherein the plurality of first capacitor plates havea material having a first work function on both a first side and asecond side thereof, and the plurality of second capacitor plates have amaterial having the first work function on a first side thereof and amaterial having a second work function different than the first workfunction on a second side thereof.
 15. The MEMS motion sensor of claim10, further comprising a comparator having a first input coupled to thecapacitor and a second input coupled to the reference capacitor and anoutput for providing a differential output signal.
 16. A method fordetecting motion through a micro-electromechanical sensor (MEMS)comprising the steps of: providing a capacitor having first and secondcapacitor plates spaced from one another, the first and second capacitorplates having different work functions and being electrically connectedwith each other, wherein the plates are movable with respect to oneanother such that a spacing between the plates changes in response to aforce; and monitoring current through the capacitor, the current beingrepresentative of a rate of change in the spacing between the plates ata given time.
 17. The method of claim 16, further comprising the step ofproviding a second capacitor having third and fourth capacitor platesspaced from one another, the third and fourth capacitor plates havingdifferent work functions, wherein the third and fourth capacitor platesare fixed with respect to one another such that a spacing between theplates is maintained for providing a reference current through thesecond capacitor, wherein the monitoring step comprises generating afirst voltage with the current through the capacitor, generating asecond voltage with the reference current, and comparing the first andsecond voltages.
 18. The method of claim 16, wherein the providing stepcomprises providing a plurality of the first capacitor plates and aplurality of the second capacitor plates, the plurality of first andsecond capacitor plates being arranged in an interdigitated relationshipto form a plurality of capacitors, the plurality of first capacitorplates having a material having a first work function on both a firstside and a second side thereof, and the plurality of second capacitorplates have a material having the first work function on a first sidethereof and a material having a second work function different than thefirst work function on a second side thereof, wherein the monitoringstep comprises monitoring a sum of the current through the plurality ofcapacitors.
 19. A micro-electromechanical apparatus for detectingmechanical displacement comprising: a capacitor having first and secondcapacitor plates spaced from one another, the first and second plateshaving different work functions and being electrically connected witheach other, wherein the plates are movable with respect to one anothersuch that a spacing between the plates changes in response to a force, acurrent through the capacitor being representative of a rate of changein the spacing between the plates at a given time; and a secondcapacitor having third and fourth capacitor plates spaced from oneanother, the third and fourth capacitor plates having different workfunctions, wherein the third and fourth capacitor plates are fixed withrespect to one another such that a spacing between the plates ismaintained in response to the force to provide a reference currentthrough the second capacitor.
 20. The apparatus of claim 19, wherein thecapacitors are connected in parallel with one another between two powersupply nodes, each capacitor being coupled in series with a resistiveelement, the apparatus further comprising a comparator having a firstinput coupled to a first node intermediate the capacitor and therespective resistive element in series therewith and having a secondinput coupled to a first node intermediate the second capacitor and therespective resistive element in series therewith, the comparator havingan output for providing a differential output signal.
 21. Amicro-electromechanical system (MEMS) comprising: a fixed supportmember; a deflection member attached to the support member so as todeflect relative to the support member; a first capacitor plateconnected to the fixed support member; a second capacitor plateconnected to the deflection member, the second capacitor plate beingparallel to and spaced from the first capacitor plate, the firstcapacitor plate and the second capacitor plate forming a capacitor, thefirst and second capacitor plates having different work functions andbeing electrically connected with each other to provide a currentthrough the capacitor, the current through the capacitor being based onthe deflection of the deflection member; and a second capacitor havingthird and fourth capacitor plates spaced from one another, the third andfourth capacitor plates having different work functions, wherein thethird and fourth capacitor plates are fixed with respect to one anothersuch that a spacing between the plates is maintained for providing areference current through the second capacitor.
 22. The MEMS of claim21, further comprising a comparator having a first input coupled to thecapacitor and a second input coupled to the second capacitor and anoutput for providing a differential output signal.